Add a service template that adds the machine tag given as its instance
name to the host's machine tags. Whenever an instance is pulled in, the
tag is added; the operation is idempotent.
It talks to hostnamed over Varlink (which is socket-activated), so it
works very early at boot, before the system bus is up. It is ordered
after local-fs.target so that /etc/machine-info can be written, requires
hostnamed's socket and the local filesystems to be available, and does
not run in the initrd.
